The present invention relates to a process for producing an epitaxial layer of gallium nitride (GaN) as well as to the epitaxial layers of gallium nitride (GaN) which can be obtained by said process. Such a process makes it possible to obtain gallium nitride layers of excellent quality.
It also relates to the short-wavelength optical devices or the high-power high-frequency electronic devices provided with such an epitaxial gallium nitride layer.
It relates in particular to optoelectronic components formed on such gallium nitride layers.
Processes are known for obtaining relatively thick GaN layers, for example from 100 to 200 micrometers. The method commonly used is chloride and hydride vapor phase epitaxy (HVPE). Either sapphire substrates or GaN layers on sapphire 200 micrometers in thickness are used, these layers being fabricated by OrGaNoMetallic Pyrolisis Vapor Phase Epitaxy (OMVPE). However, the crystal lattice parameter mismatch between sapphire and GaN is such that the build-up of stresses in the layers results in cracks and prevents the sapphire substrate from being removed. All the experimental innovations (treatment of the surface of the sapphire at the start of growth with GaCl, deposition of a ZnO interlayer) have not made it possible to solve this problem. At the present time, the relatively thick GaN layers have a double X-ray diffraction (DXD) line width of the order of at best 300 arcsec, which means that the crystallographic quality does not exceed that of the layers formed by OMVPE or by molecular beam epitaxy (MBE).
In other words, no potential sapphire, ZnO, 6H-SiC or LiAlO2 substrate is ideal for nitride epitaxy (excessively high lattice mismatch and thermal expansion coefficient mismatch, thermal instability).
Moreover, the lasing effect (by optical pumping) on GaN has been known for a long time. Although diode lasers based on III-V nitride have been produced, the crystal quality of the nitride layers constituting the structure of these lasers is very average. Dislocation densities ranging from 109 to 1010 cmxe2x88x922 have been measured.
In fact, the defects associated with the formation of relatively thick epitaxially grown GaN layers indicated above have considerably slowed down the development of diode lasers provided with such layers: high residual n, absence of single crystals and of suitable substrates, impossibility of producing p-doping.
The publication by D. Kalponek et al., Journal of Crystal Growth, 170 (1997) 340-343 mentions the localized nitride growth in apertures formed in a mask so as to form pyramidal structures. However, this document neither describes nor suggests the formation, by coalescence, of features or islands of smooth gallium nitride layers.
The publication Y. Kato, S. Kitamura, K. Hiramatsu and N. Sawaki, J. Cryst. Growth, 144, 133 (1994) describes the selective growth of gallium nitride by OMVPE on sapphire substrates on which has been deposited a thin gallium nitride layer masked by an SiO2 layer etched so as to reveal continuous bands of gallium nitride.
However, the localized epitaxy thus carried out involves neither the lateral growth nor the growth anisotropy as will be described below.
The document EP 0,506,146 describes a process for local and lateral growth using a mask, shaped by lithography, to localize the growth. The examples of smooth layers relate in no case to gallium nitride. These examples mention GaAs homoepitaxy on a GaAs substrate and InP homoepitaxy on an InP substrate.
The object of the process according to the invention is to obtain crystalline layers allowing the production of optoelectronic devices (especially diode lasers) having life times and performance characteristics which are superior to those obtained previously.
The inventors have found that the treatment of a substrate by deposition of a suitable dielectric followed by deposition of gallium nitride, which is itself followed by thermal annealing, causes the formation of gallium nitride islands which are virtually defect-free.
The coalescence of such islands caused by the heat treatment results in a gallium nitride layer of excellent quality.
The invention relates firstly to a process for producing a layer of gallium nitride (GaN), characterized in that it comprises the deposition on a substrate of a dielectric layer functioning as a mask and the regrowth of gallium nitride on the masked substrate under epitaxial deposition conditions so as to induce the deposition of gallium nitride features and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features. The term xe2x80x9cislandsxe2x80x9d instead of xe2x80x9cfeaturesxe2x80x9d may also be employed.
The substrate generally has a thickness of a few hundred micrometers (in particular, approximately 200 micrometers) and may be chosen from the group consisting of sapphire, ZnO, 6H-SiC, LiAlO2, LiGaO2 and MgAl2O4. The substrate is preferably treated beforehand by nitriding.
Preferably, the dielectric is of the SixNy type, especially Si3N4. SiO2 may also be mentioned, but other well-known dielectrics could be used. The deposition of the dielectric is carried out in the gallium nitride growth chamber from silane and ammonia.
Preferably, the carrier gas is an N2/H2 mixture.
According to a first embodiment, the dielectric layer is an atomic monolayer, or a cover of the order of the atomic plane.
Next, epitaxial regrowth on the substrate is carried out using OMVPE. Regular features or islands develop. Examination in a high-resolution electron microscope shows that the GaN dislocation density in the regular features or islands, which has therefore grown without heteroepitaxial strains, is very much less than that produced by the direct deposition of gallium nitride on the substrate. Thus, the GaN growth, which takes place laterally in the [10{overscore (1)}0] directions on a dielectric surface, and therefore without being in epitaxial relationship with the sapphire substrate, results in much better GaN crystal quality than the usual processes. After said features have been obtained, the growth may be continued, either using OMVPE or HVPE. Growth takes place laterally, until coalescence of the islands. These surfaces resulting from the coalescence of islands exhibit crystal quality superior to the layers grown heteroepitaxially on sapphire.
The gallium nitride deposition is generally carried out in two steps. A first step, at a temperature of approximately 600xc2x0 C. for the deposition of a buffer layer, from which the GaN features will emerge, then at a higher temperature (approximately 1000-1100xc2x0 C.) for the growth of an epilayer from said features.
According to a second embodiment, the invention relates to a process characterized in that the dielectric layer is etched, so as to define apertures and to expose the facing regions of the substrate, and gallium nitride is regrown under epitaxial deposition conditions on the masked and etched substrate so as to induce the deposition of gallium nitride features on the facing regions and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features.
According to a third embodiment, the invention relates to a process for producing an epitaxial layer of gallium nitride (GaN), comprising the deposition of a thin gallium nitride layer on a substrate characterized in that:
a dielectric layer is deposited on said thin gallium nitride layer;
the dielectric layer is etched so as to define apertures and to expose those regions of said thin gallium nitride layer which face them;
gallium nitride is regrown under epitaxial deposition conditions on the expitaxially grown, masked and etched substrate so as to induce the deposition of gallium nitride features on the facing regions and the anisotropic and lateral growth of said features, the lateral growth being continued until coalescence of the various features.
The process according to the invention is noteworthy in that it limits the density of defects generated by the parameter mismatch between GaN and the substrate using a method which combines localized epitaxy, growth anisotropy and lateral growth, thereby limiting the epitaxial strains.
The process according to the invention makes use of deposition and etching techniques well-known to those skilled in the art.
According to the second embodiment, a dielectric a few nanometers in thickness is deposited in the growth chamber. Next, by photolithography, apertures are defined in the dielectric layer, thus exposing micrometric regions of the surface of the substrate.
Regrowth on the masked and etched substrate is carried out using OMVPE.
The substrate generally has a thickness of a few hundred micrometers (in particular, approximately 200 micrometers) and may be chosen from the group consisting of sapphire, ZnO, 6H-SiC, LiAlO2, LiGaO2 and MgAl2O4.
Preferably, the dielectric is of the SixNy type, especially Si3N4. SiO2 may also be mentioned, but other well-known dielectrics could be used. The dielectric is deposited in the gallium nitride growth chamber from silane and ammonia directly on the substrate, as described above.
According to the third embodiment, the gallium nitride is firstly grown epitaxially on the substrate by OMVPE. The deposition of a dielectric a few nanometers in thickness is then carried out in the growth chamber. Next, by photolithography, apertures are defined in the dielectric layer, thus exposing micrometric regions of the gallium nitride surface.
Regrowth on the epitaxially grown, masked and etched substrate is carried out using OMVPE.
The substrate generally has a thickness of a few hundred micrometers (in particular, approximately 200 micrometers) and may be chosen from the group consisting of sapphire, ZnO, 6H-SiC, LiAlO2, LiGaO2 and MgAl2O4.
Preferably, the dielectric is of the SixNy type, especially Si3N4. SiO2 may also be mentioned, but other well-known dielectrics could be used. The dielectric is deposited in the gallium nitride growth chamber from silane and ammonia directly after the gallium nitride deposition.
The etching of the dielectric is in particular carried out by photolithography.
Discrete apertures, or apertures in the form of stripes, are defined in the silicon nitride layer, thus exposing the gallium nitride surface on a micrometric feature. The apertures are preferably regular polygons, especially ones of hexagonal shape. Advantageously, the discrete apertures are inscribed in a circle of radius of less than 10 micrometers, whereas the apertures in the form of stripes have a width of less than 10 micrometers, the length of the stripes being limited only by the size of the substrate.
Spacing of the apertures is regular and must allow, localized gallium nitride epitaxy followed by anisotropic and lateral growth.
In general, the portion of exposed area of substrate or of gallium nitride to the total area of the substrate is between 5 and 80%, preferably between 5 and 50%.
It has been found that gallium atoms are not deposited on the dielectric and that, in other words, this etched dielectric surface allowed the gallium atoms to concentrate on the apertures.
Next, regrowth on the substrate is carried out using OMVPE. Regular features or islands develop. Examination in a high-resolution electron microscope shows that the GaN dislocation density in the regular features or islands, which has therefore grown without heteroepitaxial strains, is very much less, in the case of the third variant, than that existing in the first GaN layer. Thus, the GaN growth, which takes place laterally in the [10{overscore (1)}0] directions on a dielectric surface, and therefore without being in epitaxial relationship with the sapphire substrate, results in much better GaN crystal quality than the usual processes. After obtaining an array of regular features, the growth may be continued, either by OMVPE or by HVPE. It is carried out laterally, until coalescence of the islands. These surfaces resulting from the coalescence of islands exhibit superior crystal quality to the layers grown heteroepitaxially on sapphire.
The novelty of the process therefore consists in using the growth anisotropy to induce lateral growth, going as far as coalescence, and thus in obtaining a continuous strain-free GaN layer. The lateral growth takes place from gallium nitride features or islands having reduced defect densities, said features being obtained by localized epitaxy.
According to a variant, the epitaxial regrowth is carried out using undoped gallium nitride.
According to another variant, the epitaxial regrowth is carried out using gallium nitride doped with a dopant chosen from the group consisting of magnesium, zinc, cadmium, beryllium, calcium and carbon, especially with magnesium. This is because it has been found that the doping of gallium nitride with a doping agent, especially magnesium, modified the GaN growth mode and resulted in a relative increase in the growth rate in the  less than 10{overscore (1)}1 greater than  directions with respect to the growth rate in the [0001] direction. Preferably, the dopant/Ga molar ratio is greater than 0 and less than or equal to 1, advantageously less than 0.2.
According to another advantageous variant, the epitaxial regrowth is carried out in two steps.
Firstly, undoped gallium nitride is deposited on the etched dielectric or with a thickness of the order of one {dot over (a)}ngstrom, under vertical growth anisotropy conditions, and then gallium nitride continues to be deposited in the presence of a dopant in order to favor the lateral growth resulting in coalescence of the features.
The invention also relates to the epitaxially grown gallium nitride layers, characterized in that they can be obtained by the above process. Advantageously, these layers have a defect density of less than those obtained in the prior art, especially less than approximately 109cmxe2x88x922.
Preferably, the epitaxial layer has a thickness of between 1 and 1000 micrometers and optionally in that it is self-supported after the substrate has been separated.
The invention finds particularly advantageous application in the production of diode lasers provided with an epitaxial gallium nitride layer described above.